Anodic aluminum oxide (AAO) has been previously investigated and utilized in numerous products. Recently, interest in nanoscale materials and their outstanding properties has rapidly increased. AAO is one of the most popular nanomaterials with various applications including: molecular separation, catalysis, energy generation and storage, electronics and photonics, sensors and biosensors, drug delivery, and template synthesis. Material fabrication of AAO is based on an inexpensive electrochemical anodization with a self-ordering process of nanopores. AAO fabrication does not require any lithography or templating, and the process results in well ordered and size controlled nanopores. The density and size of the nanopores can be controlled to a certain degree during fabrication. Recent research into AAO is characterized by a large number of innovations, in particular with regard to controlling and designing intricate structural features, such as modulated, branched, and multilayered pore architectures.
Interest in utilizing AAO technology to improve the efficiency of optoelectronic devices, such as light emitting diodes, is increasing. In particular, group III nitride (AlGaN)-based light emitting diodes (LEDs) have attracted considerable attention as components in solid-state lighting. However, for light emitting diodes emitting in the ultraviolet range, light extraction efficiency as well as an overall efficiency of such devices must be improved to make them a viable alternative to UV emitting lamps, such as mercury based emitters. In optoelectronic devices, AAO technology has been utilized, for example, as a mask to pattern a substrate or a semiconductor layer in preparation for epitaxial growth. For example, in one approach, an AAO mask was formed over an aluminum nitride (AlN) buffer layer grown on a sapphire substrate. The AAO mask was generated by first depositing a few micron thick layer of aluminum over the AlN layer (which was deposited in turn using reactive sputtering). Subsequently, a two-step anodization was applied to achieve a nanoporous alumina layer. Using the nanoporous AAO template as a mask, the substrate was etched using argon (Ar) plasma until all of the AAO mask was etched away to form a nanoporous AlN layer. The nanoporous AlN layer was used to grow a gallium nitride (GaN) based semiconductor heterostructure, which was used to fabricate a light emitting diode (LED).
In another approach, AAO was used as a mask for etching nano-holes in a surface of a group III nitride-based LED to increase extraction efficiency of the LED. A similar patterning technique has been applied to achieve nano-patterning on a large surface area of a GaN-based LED chip to improve the light extraction efficiency. In this case, the pore spacing was modulated from 100 nm to 400 nm to achieve optimal performance. When operated at twenty milliamps (mA), a light output power enhancement of 42% was obtained from the p-side surface nano-patterned LEDs compared to conventional LEDs fabricated on the same wafer. This approach offers a potential technique for fabricating nanostructures on GaN-based LEDs with the advantages of large area, rapid process, and low cost.
Still another approach proposes to use an AAO film as a dry etching mask to transfer nanoporous patterns onto a sapphire substrate. Subsequently, a semiconductor heterostructure was grown on the sapphire substrate to form a light emitting diode. Epitaxial growth on a patterned substrate allows for reduction in threading dislocations in the semiconductor layers. In addition, air voids formed from the patterning procedure and subsequent growth can effectively reflect photons downward toward the top portion of the LED, thus increasing an overall extraction efficiency of the LED.
Other uses of AAO have been explored. In one approach, AAO was used as a shadow mask to etch n-type semiconductor layers prior to deposition/epitaxial growth of subsequent semiconductor layers during the fabrication of an LED. While this is an advantageous process, it is not the most technological or most cost effective procedure, since it requires at least two metal organic chemical vapor deposition (MOCVD) steps separated by AAO anodization and etching. Splitting MOCVD into two separate steps is ineffective, and etching is a technologically inefficient step. Furthermore, etching can provide for high number of defects in the underlying layer, which can result in damaging effects. In another approach, AAO anodization and etching are performed to pattern the substrate. While this procedure is technologically more amiable, substrate patterning through etching is still a relatively complex step.